Small interfering RNAs (siRNAs) have gained much attention for their powerful ability to suppress gene expression. Introduction of double-stranded RNA (dsRNA), that are homologous in sequence to a gene, has proven to suppress that gene's expression through a process known as RNA interference (RNAi); this process post-transcriptionally silences a gene through mRNA inhibition or degradation, siRNAs appear to suppress gene expression without producing a non-specific cytotoxic response. The siRNA mediated gene silencing can be achieved by transfection of cells with synthetic siRNAs, siRNAs produced by in vitro T7 based transcription system, and plasmid based DNA vectors. However, improved vector systems are needed, where the expression of siRNA from double-stranded oligonucleotides can be regulated by on and off switches. The development of DNA vectors, where the siRNA expression can be turned on and off at will have tremendous value in functional studies. These vectors can be used to create transgenic animals or permanent cell lines using siRNA technology. Development of knockout animals by RNAi may be more desirable because it is less time consuming than creating knockout animals by homologous recombination. Viral vectors, such as retroviral and adenoviral siRNA vectors would be useful to deliver the siRNA targets (triggers) into the cell lines, which are not normally transfected by lipid and other transfection reagents. These viral vectors would also allow high-efficiency delivery of triggers into animals. Using these vectors, we would construct a panel of siRNA vectors for systematic silencing of genes involved in cancer. The vectors developed during this study would have wide applications in functional genomics as well as identifying targets for disease prevention and drug design.